# Griffiths Page 150: Define "Pure" & "Physical" Dipoles

• ehrenfest
In summary, on page 150 of Griffith's E and M book, he discusses the concepts of "pure" and "physical" dipoles. A physical dipole refers to a pair of equal but opposite charges separated by a distance, while a pure dipole has a non-zero dipole moment but zero separation distance. In Example 3.8, Griffith's uses a point charges model to illustrate a physical configuration with a dominant dipole moment, and defines a "pure" dipole as a configuration with only a dipole moment and no higher moments. This is clearer in more advanced texts such as Corson and Lorrain.
ehrenfest
[SOLVED] Griffiths page 150

## Homework Statement

On this page, Griffith's start talking about "pure" and "physical" dipoles. Can someone define what these terms mean?

## The Attempt at a Solution

Isn't it explained pretty clearly in the same page?

A physical dipole (one with a finite separation between charges)

Where exactly are you having difficulty?

Last edited:
A physical dipole comprises a pair of equal but opposite charges $q$ separated by a vector $2a\hat r$. The dipole moment is $2aq\hat r$. By decreasing the separation distance but increasing the charge you can keep the dipole moment constant. A pure dipole has a zero separation distance but a non-zero dipole moment. Such a thing is not physically realizable.

NicolasPan
ehrenfest said:

## Homework Statement

On this page, Griffith's start talking about "pure" and "physical" dipoles. Can someone define what these terms mean?
What G means in Ex. 3.8 is that the point charges example is one physical configuration that has a dominant dipole moment. He seems to define a "pure" dipole as a configuration that has ONLY a dipole moment, and no higher moments. As he says, that point charges model is only a "pure" dipole in the limit -->0. The sphere with with cos charge distribution is a pure dipole because its potential for r>R is pure dipole.
None of this is too clear in G because he does not discuss higher dipole moments in good detail. Some things are clearer in more advanced texts.

DH and pam explained nce...

You may try to get a copy of Corson and Lorrain for more rigorous treatment of higher terms.

## 1. What is the definition of a "pure" dipole?

A "pure" dipole is a type of electric dipole where the positive and negative charges are separated by a very small distance. This results in a strong electric dipole moment and a strong electric field.

## 2. How is a "pure" dipole different from other types of dipoles?

A "pure" dipole differs from other types of dipoles in that the separation between the positive and negative charges is very small, resulting in a strong electric dipole moment. Other types of dipoles may have larger separations or may not have equal and opposite charges.

## 3. What is the significance of a "pure" dipole in physics?

"Pure" dipoles are important in physics because they are used to model and understand various phenomena, such as the behavior of molecules and the effects of electric fields. They also play a role in explaining the behavior of materials and their response to external electric fields.

## 4. How is a "physical" dipole defined?

A "physical" dipole is a type of electric dipole where the separation between the positive and negative charges is not infinitesimally small, but still significant. This results in a weaker electric dipole moment and electric field compared to a "pure" dipole.

## 5. What are some examples of "pure" and "physical" dipoles in real-world applications?

A "pure" dipole can be seen in the behavior of molecules, such as water, where the positive and negative charges of the atoms are very close together. A "physical" dipole can be seen in the behavior of antennas, where the positive and negative charges are separated by a certain distance to create an electric field for transmitting and receiving signals.

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